1. Field of the Invention
The present invention relates to film bulk acoustic wave resonator filters employing film bulk acoustic wave resonators (hereinafter referred to as FBARs) and a manufacturing method thereof.
2. Description of the Prior Art
With the proliferation of mobile communication typified by mobile telephony, there is an increasing need for radio frequency filters for several hundred MHz to several GHz. Among many types of filter technologies, a filter that is constructed from FBARs has features of excellence in the following respects: (1) higher frequency operation, (2) smaller construction, (3) temperature characteristics, and (4) high voltage immunity.
A basic FBAR structure is as follows: a piezoelectric layer is sandwiched between a bottom electrode layer and a top electrode layer and an underlayer supports this sandwich over a cavity (for example, refer to non-patent document 1). By applying an electric signal between the two electrodes isolated by the piezoelectric layer, bulk acoustic waves are excited in the piezoelectric layer. At this time, the path of the bulk acoustic waves is a diaphragm structure consisting of the sandwich and the underlayer. Because the top and bottom of the diaphragm structure are acoustically isolated by air, the bulk acoustic waves reflect from the interfaces between the diaphragm structure and the air, which prevents the acoustic energy from leaking out of the diaphragm structure.
To facilitate fabrication of a number of FBARs with different resonant frequencies on a single substrate, the following-improved FBAR structures are known: a structure in which a loading electrode is added to the bottom surface of the bottom electrode layer (for example, refer to patent document 1); a structure in which a loading layer is added to the surface of the piezoelectric layer (for example, refer to patent document 2); and a structure in which a loading electrode is added to the surface of the top electrode layer (for example, refer to patent document 3).
An FBAR filter employing FBARs for a 5 GHz, high-speed wireless LAN has lately been made in pubic (for example, refer to non-patent document 2). In this publication, only a general description is provided as: “designed topology consists of four stages of ladder type filters in which mass loads are applied to shunt resonators only.”    [Patent document 1]    JP-A No. 2002-299980    [Patent document 2]    JP-A No. 2002-299979    [Patent document 3]    JP-A No. 2002-335141    [Non-patent document 1]    1994 IEEE International Frequency Control Symposium, pp. 135-138    [Non-patent document 2]    2002 IEEE Ultrasonics Symposium, pp. 969-972